1. Field of the Invention
The present invention relates to a method for forming an optical thin film, and an optical element provided with the optical thin film. In particular, the present invention relates to a method for forming an optical thin film having good optical characteristics with respect to a light beam having a short wavelength of not more than 300 nm. The present invention also relates to an optical element which is provided with such an optical thin film, and an exposure apparatus which has such an optical element.
2. Description of the Related Art
A variety of optical thin films are applied to the optical system. The optical thin film is roughly classified into the antireflection film and the reflection film, and they are applied to the surface of the optical element such as lenses and prisms. The antireflection film is applied in order to reduce any unfavorable reflection. The reflection film is applied in order to efficiently reflect an incident light beam on the surface of an optical material so that the light amount is retained with desired optical characteristics.
In general, such an optical thin film is produced by the dry method (dry process). The dry process means the liquid-free process such as the vacuum deposition, the sputtering and CVD (Chemical Vapor Deposition). The method of the dry process is disclosed, for example, in Joy George, Preparation of Thin Films (Marcel Dekker, Inc., New York, 1992) and Francois R. Flory, Thin Films for Optical Systems (Marcel Dekker, Inc., New York, 1995). It is known that in order to obtain a film having high performance such as the wide wavelength band, the wide angular characteristics, and the desired reflection characteristics (low reflectance for the antireflection film and high reflectance for the reflection film), a plurality of coating materials having different refractive indexes may be combined and multilayered. It is also known that the larger the difference in refractive index among the coating materials to be used is, and the lower the minimum refractive index of the coating material to be used is, the more improved the optical performance is. It is also known that the number of coating layers can be decreased by increasing the difference in refractive index among the coating materials to be used and by using the coating material having a low refractive index. As a result, an optical thin film, which has the high performance with respect to the light beam in the visible region, has been obtained.
The ultralarge scale integrated circuit (ULSI) is being progressively highly integrated to have higher function year by year. At present, the printing line width of the semiconductor circuit is extremely fine and minute, i.e., 0.18 μm. In such circumstances, the high resolution and the deep depth of focus are required for the projection lens to be used for the reduction projection semiconductor exposure apparatus. The resolution and the depth of focus are determined by N.A. (numerical aperture) of the lens and the wavelength of the light beam to be used for the exposure. The finer the pattern is, the larger the angle of the diffracted light beam is. Unless N.A. of the lens is large, it is impossible to incorporate the diffracted light beam. The shorter the exposure wavelength λ is, the smaller the angle of the diffracted light beam is with an identical pattern. Therefore, it is enough that N.A. is small. The resolution and the depth of focus are expressed by the following expressions.Resolution=k1×λ/N.A.Depth of focus=k2×λ/(N.A.)2(in the expressions, k1 and k2 are proportional constants).
Therefore, it is appreciated that N.A. may be increased, or λ may be shortened, in order to improve the resolution. According to the expressions described above, it is advantageous to shorten λ in view of the depth of focus. From such a viewpoint, the wavelength of the light source is progressively shortened from the g-ray (436 nm) to the i-ray (365 nm), the excimer laser such as KrF (248 nm), ArF (193 nm) and F2 (157 nm).
When the antireflection film for the visible region is formed by means of the dry process, a variety of coating materials have been successfully used. For example, TiO2 (refractive index n=2.4 to 2.7 at λ=500 nm) has been used as the maximum refractive index material, and MgF2 (refractive index n=1.38 at λ=500 nm) has been used as the minimum refractive index material. However, when the ultraviolet light beam in the vicinity of 200 nm is used, only a few coating materials can be used, because almost all substances absorb the light beam in this wavelength region. For example, the substance, which is generally used as the maximum refractive index material, is represented by LaF3, NdF3, and GdF3 (any of the materials has a refractive index n=about 1.7 at λ=200 nm). The minimum refractive index material is represented by Na3AlF6 (refractive index n=1.36 at λ=200 nm). Therefore, the difference in refractive index between the maximum refractive index and the minimum refractive index of the coating materials used at the wavelength of 200 nm is extremely smaller than the difference in refractive index among those of the coating materials used in the visible region. Therefore, it is difficult to form an optical thin film for the ultraviolet region having high optical performance.
As described above, the coating material, which is usable in the ultraviolet region, is limited to the several types. Therefore, it is difficult to design a high performance optical thin film for the ultraviolet region as compared with the optical thin film for the visible region. In the visible region, a low reflectance antireflection film has been obtained, which has a wide band width to reduce the reflectance (for example, the reflectance is not more than 0.5% in a wavelength region of 400 nm to 800 nm). In spite of such problems that the production time is prolonged and the production cost is expensive, the high performance antireflection film as described above can be realized by laminating a large number of, i.e., eight layers, nine layers or more layers of antireflection film layers. As for the antireflection film for the ultraviolet region, no sufficiently satisfactory performance has been obtained yet for the reason as described above. However, an antireflection effect is obtained by providing the film construction having a multilayered structure composed of about ten layers in the same manner as in those for the visible region. On the other hand, it has been clarified by simulation that the antireflection effect for the ultraviolet light beam can be extremely enhanced when only the uppermost layer of the antireflection film layers is a super low refractive index film having a refractive index of not more than 1.30. Further, it is also possible to decrease the number of layers of the antireflection film as a whole. Therefore, such an antireflection film is also advantageous in production time and cost. Therefore, if a super low refractive index film having a refractive index of not more than 1.30 is formed, the resolution of the optical system is successfully improved to greatly contribute to the realization of high performance of the semiconductor-producing apparatus provided with the projection optical system for which the high resolution is required.
In order to lower the refractive index of the film, it is known that the structure of the film is not dense but porous. In general, it is defined that the film has a structure composed of a plurality of minute pores to separate accumulated solid substance. Therefore, the relationship between the filling density and the refractive index of the film is represented as follows.nf=n0×P+np×(1−p)
In the expression, np represents the refractive index of the substance (for example, air or water) with which the minute pores are filled, nf and n0 represents the real refractive index (depending on the filling density) and the refractive index of the accumulated solid material, and P represents the filling rate of the film. Further, the filling rate is defined as follows.P=(volume of solid portion of film)/(total volume of film (solid portion+minute pore portion))
According to the foregoing expression, it is understood that the refractive index nf is low when the filling density is low.
In general, the dry process such as the vapor deposition and the sputtering is appropriate in order to obtain a dense film. However, the wet process is appropriate in order to obtain a porous film. For example, Ian M. Thomas, Applied Optics, Vol. 31, No. 28 (1992), pp. 6145-6149 discloses the production of an SiO2 film having a low filling density and a high purity by means of a sol-gel process by using a raw material of an alkoxide solution of silicon. When ammonia is added as a catalyst for the hydrolysis reaction of alkoxide, it is possible to prepare minute spherical SiO2 particles having high purity. When the solution (sol solution), in which such minute particles are suspended, is applied onto a substrate surface, and the dispersion medium of alcohol is vaporized or evaporated at room temperature, then it is possible to prepare a porous SiO2 film composed of spherical SiO2 particles, i.e., an SiO2 film having a low filling density. In the case of the SiO2 film produced by the wet process, the filling rate can be changed from 1 to about 0.5. Therefore, the refractive index can be changed from 1.45 to 1.22 in the visible region. As a result, it is possible to obtain a monolayer antireflection layer having a reflectance of almost 0% on optical glass by using the low filling density SiO2 prepared by the wet process. As well-known, the antireflection film, which is composed of the low filling density SiO2 film, has high laser durability. Therefore, the antireflection film is used for the high output laser such as those used for the nuclear fusion.
The wet process is advantageous in that the refractive index of the film can be made low. However, impurities (for example, carbon) originating from the materials and the solvent are adhered to the surfaces of the synthesized minute particles. A problem arises in that the transmittance in the ultraviolet region is lowered. In order to completely remove the impurities, it is necessary to heat-treat the film at a high temperature of at least not less than 500° C. However, if the formed film is heat-treated at a temperature of not less than 500° C., it is feared that the porous film may be densified to increase the refractive index, or the film may be oxidized to lower the transmittance when the film is composed of fluoride. Further, it is also feared that the substrate may be deformed if the heat resistance is low. Therefore, it is extremely difficult to perform the heat treatment at a temperature of not less than 500° C. so that the impurities are completely removed. That is, the wet process has involved the following problem. That is, it is impossible to sufficiently increase the transmittance in the ultraviolet region, because the impurities originating from the raw materials and the solvent are adhered to the surfaces of the minute particles, and it is impossible to completely remove the impurities. The projection optical system of the reduction projection exposure apparatus for producing the semiconductor device comprises a group of several tens of lenses, in which the antireflection films are required for both surfaces of each of the lenses. Therefore, even if the absorbance of the film is slightly increased, the projection lens suffers from large loss as a whole. In order to secure a sufficient exposure amount, it is extremely important to decrease the light absorption caused by the film as possible.
SiO2 absorbs the ultraviolet light beam having a wavelength of not more than 180 nm. Therefore, the F2 excimer laser having a wavelength of 157 nm is not transmitted therethrough. Accordingly, it is necessary to use a thin film of fluoride such as MgF2 and CaF2 so that sufficient transmittance is also exhibited for the ultraviolet light beam having an extremely short wavelength as described above. Ian M. Thomas, Applied Optics, Vol. 27, No. 16 (1988), pp. 3356-3358 discloses the production of a porous thin film composed of MgF2 and CaF2 by means of the wet process. The thin film had a sufficiently high transmittance at a wavelength in the vicinity of 350 to 400 nm. However, the transmittance was suddenly lowered at a wavelength shorter than the above. The present inventors consider that the reason of this phenomenon resides in the influence of carbon originating from the material as described above.
A first object of the present invention is to provide an optical element provided with an optical thin film in which the refractive index in the ultraviolet region at a wavelength of not more than 300 nm is not more than 1.30, and the transmittance loss of ultraviolet light in the same wavelength region is not more than 0.5%, and an exposure apparatus which has such an optical element.
A second object of the present invention is to provide an optical element provided with an optical thin film which is excellent in incidence angle characteristic and which has a good reflectance with respect to each of light beams having incidence angles in a wide range, and an exposure apparatus which has such an optical element.